One way to produce a solar cell product involves depositing a thin, light-absorbing, solid layer of the material copper indium gallium diselenide, known as “CIGS,” on a substrate. A solar cell having a thin film CIGS layer can provide low to moderate efficiency for conversion of sunlight to electricity.
Making a CIGS semiconductor generally requires using several source compounds and/or elements which contain the atoms needed for CIGS. The source compounds and/or elements must be formed or deposited in a thin, uniform layer on a substrate. For example, deposition of the CIGS sources can be done as a co-deposition, or as a multistep deposition. The difficulties with these approaches include lack of uniformity, purity and homogeneity of the CIGS layers, leading ultimately to limited light conversion efficiency.
For example, some methods for solar cells are disclosed in U.S. Pat. Nos. 5,441,897, 5,976,614, 6,518,086, 5,436,204, 5,981,868, 7,179,677, 7,259,322, U.S. Patent Publication No. 2009/0280598, and PCT International Application Publication Nos. WO2008057119 and WO2008063190.
Other disadvantages in the production of thin film devices are limited ability to control product properties through process parameters and low yields for commercial processes. Absorber layers suffer from the appearance of different solid phases, as well as imperfections in crystalline particles and the quantity of voids, cracks, and other defects in the layers. In general, CIGS materials are complex, having many possible solid phases. Moreover, methods for large scale manufacturing of CIGS and related thin film solar cells can be difficult because of the chemical processes involved. In general, large scale processes for solar cells are unpredictable because of the difficulty in controlling numerous chemical and physical parameters involved in forming an absorber layer of suitable quality on a substrate, as well as forming the other components of an efficient solar cell assembly, both reproducibly and in high yield.
For example, there is a general need for the use of selenium in the processing of CIGS materials for a solar cell. The presence and concentration of selenium in annealing, for example, is a chemical parameter that should be controlled in a solar cell manufacturing process.
In another example, introducing alkali ions at a controlled concentration into various layers and compositions of a CIGS-based solar cell has not been achieved in a general way. Conventional methods for introducing sodium do not readily provide homogenous concentration levels or control over sodium location in a CIGS film. The presence and level of alkali ions in various layers is a chemical parameter that should be controlled in a solar cell manufacturing process.
A significant problem is the inability in general to precisely control the stoichiometric ratios of metal atoms and Group 13 atoms in the layers. Because several source compounds and/or elements must be used, there are many parameters to control in making and processing uniform layers to achieve a particular stoichiometry. Many semiconductor and optoelectronic applications are dependent on the ratios of certain metal atoms or Group 13 atoms in the material. Without direct control over those stoichiometric ratios, processes to make semiconductor and optoelectronic materials can be less efficient and less successful in achieving desired compositions and properties. For example, no single source compound is currently known that can be used to prepare a layer or film of any arbitrary stoichiometry from which CIGS materials can be made. Compounds or compositions that can fulfill this goal have long been needed.
What is needed are compounds, compositions and processes to produce materials for photovoltaic layers, especially thin film layers for solar cell devices and other products.